Problems caused by feedback either acoustic or structure-borne are a familiar problem within audio, broadcasting or telecommunication.
The response of most microphones is diminished by vibrations picked up from the surrounding structure. In the case of a table microphone, this would be vibrations from the table itself. Someone working on the table may create sounds, knocking, tapping the pencil, moving cups etc., that can find their way to the microphone via vibrations. The airborne sound may also excite the surrounding structure and any resonances in this structure may be coupled to the microphone, resulting in transient and/or harmonic distortion.
In integrated video or telephone conferencing systems, vibrations (sound) from the built in loud speaker, would in addition to going through the air, also go directly to the microphone. That is because the cabinet also starts to vibrate in phase with the loud speaker. In conferences where a tabletop microphone is used and particularly where several participants take part in the conference, experience has shown that the participants will be disturbed by noise due to participants that move things on the table or tap on the table with their fingers, a table that often holds the microphone hence you will have, as mentioned above, a coupling between the microphone and in this example the table.
Undesirable coupling between loudspeaker and microphone in teleconference equipment is normally compensated by using an acoustic echo canceller. Such echo cancellers are normally based on a linear model, as the sound propagating through the air is fairly linear. However, vibrations coupled through the cabinet may have larger portions of harmonic distortion, reducing the echo canceller's performance, and may require a more complicated and expensive echo canceller. Also an echo canceller in setups with no or very low mechanical coupling (as in the case of widely separated loudspeaker and microphone) may suffer from such harmonic distortion, as the airborne sound may excite the structure surrounding the microphone.
To increase the performance of the microphone it is therefore essential to mechanically isolate the microphone from the surrounding structure in order to prevent any vibration from entering into the microphone element.
There are many well known techniques addressing the problems of isolating the noise source and the desired signal source. Suppression of unwanted couplings, in particular mechanical couplings, by isolating the sound recording and sound producing source mechanically is one. Traditionally done by e.g. for a record player using floating suspension, traditionally with the use of springs, for microphones and loudspeakers it is known to use elastic suspension for suppression of mechanical coupling. These elastic suspensions can be rubber bands forming the suspension between a microphone or a loudspeaker and its mounting device.
From U.S. Pat. No. 4,199,667 it is known a microphone where the microphone element is mechanically suspended inside a housing. The weight and the properties of the resilient material used to hold the microphone element is known, hence, the natural swinging frequencies is known and this is used to compensate for vibrations that might excite the element. Another way to compensate for the vibration generated noise in the signal is disclosed in U.S. Pat. No. 6,226,386. Here the microphone includes an oscillation-detecting device inside the microphone case. The information from this device is electronically processed and then used to modify the signal from the microphone element in order to compensate for the noise. Yet another way of solving the problem is to actually put the whole microphone case in some kind of flexible cradle, as is done in U.S. Pat. No. 4,514,598.
The problems of vibration are well known by the manufacturers of microphones, therefore these manufacturers have developed microphones that are more, but not completely, immune against vibrations. Lighter/thinner membranes are commonly used, as well as more tension in the membrane. These techniques often have drawbacks, as more expensive production or less sensitivity to airborne sound. In addition, manufacturers recommend rotating the microphone, to avoid vibrations normally (90 degrees) to the membrane. This latter technique only works if there exists a primary “plane” of vibrations.
In other applications, suppression of vibrations is achieved by gyroscopic suspension, fluid filled suspension or magnetic levitation. In the latter case the object that should be isolated from the surrounding structure will be placed on a diamagnetic member levitated relative to permanent magnets. Possible magnetic levitation systems are disclosed in e.g. U.S. Pat. No. 3,428,370 and U.S. Pat. No. 3,597,022.